Narrow band-gap donor–acceptor copolymers based on diketopyrrolopyrrole and diphenylethene: Synthesis, characterization and application in field effect transistor
Graphical abstract
We designed and synthesized three diketopyrrolopyrrole based polymers with varying donor groups of furan, thiophene and phenyl and then copolymerized with diphenylethene to achieve high mobility. We also oxidized the comonomer of diphenylethene to prove that flexible and rotatable comonomer plays an important role in lowering the band-gap and increasing the coplanarity.
Introduction
In recent years, donor–acceptor (D–A) π-conjugated polymers with narrow-band-gap have emerged as promising candidates for optoelectronic devices [1], [2], [3], [4], [5], [6], [7], [8]. Diketopyrrolopyrrole (DPP) was mostly used as an electron acceptor to form D–A copolymers for its highly conjugated lactam structure, electron withdrawing capability and a high degree of planarity [9], [10]. The polymers involving DPP units have showed enhancing intramolecular charge transport via extended delocalized π-electron distribution along the backbone [11], [12], [13], [14], [15]. The low band-gap which originates from coplanarity and well conjugated structure of DPP based copolymers makes these polymers as the key materials for semiconducting application [16], [17], [18], [19], [20].
Molecular packing of donor–acceptor has a great effect on the performance of electronic device [21], [22], [23], [24], [25]. By carefully selecting the electron donating groups on the lactam ring or the comonomer, its highest occupied molecular orbitals (HOMO) and lowest occupied molecular orbitals (LUMO) can be tuned which leads to innovative low band-gap materials [26], [27]. Various electron donating groups, such as thiophene, phenyl, selenophene and furan have been employed in DPP based copolymers [28], [29], [30]. DPP based copolymers can be prepared with the desired comonomer via either Suzuki coupling or Stille coupling [31], [32], [33], [34], [35]. Variation of the comonomer provides vast possibilities with different unit combination which may cause promising properties for semiconducting devices [36], [37], [38]. The enthusiasm towards various DPP based polymers accelerates the development of many applications in the polymer solar cells (PSCs) and field effect transistors (FETs) [39], [40], [41], [42], [43]. For instance, Sonar and co-workers have reported three DPP based copolymers using furan as the donor group and alternatively using phenylene, naphthalene or anthracene as donor comonomer blocks which finally achieved a mobility of 0.11 cm2 V−1 s−1 [44]. Shahid and co-workers have reported the synthesis of two alternating copolymers of thiophene and selenophene flanked DPP with dithienothiophene which exhibited great charge transport properties with high mobility up to 0.23 cm2 V−1 s−1 [29]. Yun and co-workers have reported that by introducing nitrile group as an electron withdrawing unit and vinyl as linkage in the DPP-based copolymer, a high mobility of 7.0 cm2 V−1 s−1 was obtained when applied in field-effect transistors [45].
We are interested in systematically investigating the effects of different flanking block and comonomers on lowering the band gap. Here, furan, thiophene and phenyl were used to attach on the DPP electron accepting moiety and then copolymerized with diphenylethene units which were shorted as PFDPP, PTDPP and PPDPP. Diphenylethene have been used in many materials with high charge carrier mobilities [46], [47]. The chemical structures of three DPP based copolymers PFDPP, PTDPP and PPDPP are shown in Scheme 1. The values of aromatic resonance energy for furan, thiophene and phenyl are 0.70 eV, 1.26 eV and 1.56 eV, respectively. The electron donating strength suggests the effective conjugation length and enhancement of planarity which would decrease the band-gap of organic semiconducting devices [48]. With diphenylethene units, DDP derivatives provide a longer effective conjugation length and promote a more delocalized HOMO distribution along the backbone which would enhance the intermolecular charge-carrier hopping [35]. To further study the electron donating comonomers, diphenylethene was oxidized to become a rigid structure. The three oxidized copolymers O-PFDPP, O-PTDPP and O-PPDPP are shown in Scheme 1. Lei and co-workers have substituted the donor part of conjugated polymers which showed the rotatable donor groups provided better FET performance than the rigid donor groups [24]. Here we demonstrate that comonomer with rigid structure leads to a larger band gap due to worse π–π stacking and intermolecular interaction which originates from the increasing torsional angle. The copolymers were characterized by nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), UV–vis absorption and cyclic voltammetry (CV). PFDPP, PTDPP and PPDPP based electric double layer transistors exhibited a p-channel performance with high mobilities. The peak mobility value of 2.36 cm2 V−1 s−1 can be reached when PFDPP was applied to field effect transistors.
Section snippets
Materials and instruments
Unless otherwise stated, all of reagents and chemicals used were purchased from Aldrich or Sinopham Chemical Reagent Company.
1H NMR and 13C NMR spectra were measured on a Bruker AVANCE III HD 300 MHz. FT-NMR spectrometer in dimethyl sulfoxide-d6 and deuterated chloroform (CDCl3). Chemical shifts are in ppm units using tetramethylsilane (TMS; δ = 0) as an internal standard. The samples were analyzed by AB SCIEX 5800 matrix assisted laser desorption ionization-time of flight mass spectrometer
Results and discussion
The synthetic route of the monomers and target polymers is clearly outlined in Scheme 1. Monomer 1, monomer 2 and monomer 3 were synthesized through the condensation reaction [49]. To increase the solubility and film-forming ability, two octyl groups were attached at the 2, 5 position of the DPP moiety. It was necessary to improve the solubility of these DPP based copolymers since they are poorly soluble in common organic solvents which results from strong hydrogen-bonding structure and π–π
Conclusion
We have used Suzuki coupling to synthesize three new polymers, PFDPP, PTDPP and PPDPP with the same comonomer diphenylethene while substituted the electron donating moieties with furan, thiophene and phenyl. Furan based DDP copolymer exhibits the lowest band-gap and the highest mobility. The results provide insight into the role of the donor units and comonomers in determining the electrical properties and structure of DPP based copolymers. The influence of comonomer was studied by oxidizing
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (21274027, 20974022) and the Innovation Program of Shanghai Municipal Education Commission (15ZZ002). The synchrotron-based 2D GIXRD measurements at beamline BL14B1 were supported by the Shanghai Synchrotron Radiation Facility (15ssrf00474).
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